Well there is so much to choose from, it really depends on the role you want to play.
If you want something relatively quick/easy to get (that you can get kinda early on), I suggest Ember. If you are willing to work for it a bit, Rhino also might be a good choice for you (but IIRC he's mastery level locked, so you'll need to level up weapons and warframes to raise your mastery level)
Ember is a pretty strong but squishy caster frame. Host both nuking and CC abilities (but is strongest for nuking)
Rhino is a very good tank and CC frame
Trinity is strong support
Oberon is mix of support and CC/offense
Mag and Volt are both decent for the most part. Both are caster frames.
Saryn is a tanky caster frame for the most part
Valkyr is a bit more melee focused than Excal, and also more tanky. She's my current main
Nova is CC and nuker frame
Frost is a very good defensive frame. Has both nuking and CC abilities.
Loki and Ash are stealth frames, with Ash being more on the nuking side, and Loki on the CC side
And there are so many more, but many of them are VERY hard or time-consuming to get.
Also, Volt, Banshee, Zephyr, and Wukong are all obtainable in the Tenno Research room which can be built in Clan Dojos. In other words you'll need to be a part of a clan with those Warframes researched to be able to get them (assuming you don't want to spend platinum/IRL money), and Zephyr and Wukong are quite expensive to build.
Poll